Methacrylic resins are being used for automobile parts, lighting equipment, various panels and the like, because of their excellent transparency, weather resistance, moldability and other properties. However, methacrylic resins generally have insufficient impact resistance, so that many propositions have hitherto been made in order to improve their impact resistance.
For example, Japanese Patent Publication No. 27576/'80 has proposed a technique in which the impact resistance of hard resins such as methacrylic resins is improved by adding thereto a multilayered polymer having a specific basic structure consisting of hard-soft-hard three layers. Moreover, Japanese Patent Publication No. 88903/'93 has proposed a thermoplastic acrylic resin composition containing a hard-soft-hard polymer having a specific structure and specific properties, with a view to improving impact resistance, transparency, and the lowering the variation of haze with temperature. Moreover, Japanese Patent Laid-Open No. 230841/'87 has proposed a method for improving the balance of transparency, gloss, rigidity and impact resistance in multilayered graft copolymers having a semisoft-soft-hard three layers structure. Furthermore, Japanese Patent Laid-Open No. 93056/'94 has proposed a method for improving the Izod impact strength and resistance to impact whitening of a hard-soft-hard three layers emulsion polymer by using an arylalkyl (meth)acrylate in place of styrol (styrene) serving as a component of the elastomer layer, so as to create a well-defined interface between the hard core and the elastomer layer.
However, the term "impact resistance" as used in these propositions exclusively means the impact strength (Izod impact strength) of a specimen having a notch (or cutout). Although this is a common method for evaluating the impact resistance of molded articles having a complicated shape with corners or the like, no particular consideration is given to falling-ball or falling-weight impact strength which is an index to the impact strength of molded articles used chiefly in the form of flat plates such as signboards and various covers. Accordingly, there is room for improvement.
Japanese Patent Publication No. 11970/'85 has proposed a method for improving falling-weight impact strength, solvent resistance and other properties by disposing an intermediate layer respectively between adjacent layers of a specific hard-soft-hard three-layer structure and controlling the particle diameter. Moreover, Japanese Patent Publication No. 17406/'85 has proposed a method for improving falling-weight impact strength, solvent resistance and other properties by controlling the molecular weight of the third layer in a specific hard-soft-hard three-layer structure. However, the degree of improvement brought about by these methods is less than satisfactory, and still leaves room for improvement.
Furthermore, Japanese Patent Laid-Open No. 17654/'93 has proposed a method for improving the gloss, transparency, falling-weight impact strength and processability of a multilayered graft copolymer having a semisoft-soft-hard three layers structure in which, during preparation of the graft copolymer, the addition and polymerization of the monomer to form the third layer is started at the time when the polymer of the second layer has reached a specific degree of polymerization. However, this proposition still fails to achieve sufficiently high transparency and falling-weight impact strength, and resistance to impact whitening also remains to be improved.